The amount of energy a particular type of forage contributes to a ruminant diet is perhaps the single most important factor in predicting animal performance. Ruminant diets containing high-energy forages yield faster weight gains and greater milk production. However, accurately, precisely, and quickly measuring the energy content of forage currently is not possible.
In the past, empirical equations were used to predict forage energy content from a single analyte, such as acid detergent fiber (ADF) or crude protein (CP). While these empirical equations are generally accurate for large sample sizes, they yield imprecise results. In short, when examining a large database of forage energy contents predicted by an empirical equation, the empirical equation accurately predicts the average of the database. But the equation will not precisely predict the energy content of any single forage within the database. To be of practical value, a forage testing system must be able to determine (accurately and precisely) the energy content of any single forage.
One approach to measuring the energy content of forage is to measure the principal components in the forage that contribute energy, give each component a digestion coefficient, multiply each component by its respective digestion coefficient, and add the products together. (See Weiss, W. P. (1996) “Estimating Available Energy Content of Ruminant Feeds,” Proc. California Nutrition Conference, 1-11, Fresno, Calif.) The disadvantage of this approach is that extensive laboratory measurements are needed. Four principal components need to be accurately and precisely measured: crude protein (CP), neutral detergent fiber (NDF), fat, and non-fiber carbohydrate (NFC). The digestion coefficients assigned to CP, fat, and NFC are well defined by research. (See Weiss, W. P. (1993) “Prevailing concepts in energy utilization by ruminants. Predicting energy values of feeds,” J. Dairy Sci. 76:1802-1811.) However, the digestion coefficient for NDF (i.e., neutral detergent fiber digestibility, NDFD) is not well defined by research and is not easily determined in the lab.
NDFD is one of the more difficult assays to conduct in the laboratory. Most laboratories cannot conduct the assay because an in vitro NDFD laboratory procedure requires rumen fluid from a live cannulated cow. Conventionally, forage NDFD is measured via two methods: In the first method, forage samples are placed in small Dacron bags and inserted into the rumen of a cow via a ruminal cannula. The amount of NDF prior to ruminal incubation is compared to the amount of NDF remaining after ruminal incubation. NDFD is then calculated from the “before digestion” and “after digestion” NDF values to arrive at a value for NDF digestibility. This approach is generally referred to as the in situ method. While the in situ method is a viable method for estimating NDFD, not every lab can have a number of cannulated cows on hand to be used for this purpose. Also, there is large, uniform database of NDFD values determined via the in situ method.
In the second conventional method, an in vitro approach is taken. The basic process is as follows (see Goering & Van Soest (1970) “Forage Fiber Analyses (Apparatus, Reagents, Procedures, and Some Applications),” Agric. Handbook No. 379, pp 8-11, ARS-USDA, Washington, DC; see also Van Soest, Robertson & Lewis (1991) “Methods for Dietary Fiber, Neutral Detergent Fiber, and Nonstarch Polysaccharides in Relation to Animal Nutrition,” J. Dairy Sci. 74:3583-3597): Feed is weighed into a glass flask. Buffers, macro- and micro-minerals are then added to the flask, along with rumen fluid extracted from a cow fit with a ruminal cannula. The forage, buffers, and rumen fluid are then incubated in a water bath in an anaerobic environment (carbon dioxide) at a cow's body temperature (102° F./39° C.) for 48 hours. After the 48 hours has passed, the flask containing the forage, buffers, and rumen fluid is removed from the water bath and the remaining solution is refluxed in neutral detergent solution for 1 hour. After refluxing, the remaining solution is filtered and the NDF that resisted digestion by rumen bacteria is retained on the filter. The digestible NDF values are then calculated by difference.
Only minor changes have been made to the in vitro NDFD assay since it was first put forth in 1970. As of 2001, the National Research Council recommended using a 48-hour incubation period. See “Nutrient Requirements for Dairy Cattle,” 7th Revised Ed., Subcommittee on Dairy Cattle Nutrition, Committee on Animal Nutrition, National Research Council, Nat. Acad. Sci., Washington, DC.
The conventional in vitro NDFD assay thus suffers from two very distinct drawbacks: (1) it is slow: 48 hours; and (2) it is unacceptably imprecise because it uses rumen fluid that is not standardized. In short, the conventional in vitro assay requires using rumen fluid, which differs in its bacterial content and fiber digestion activity from cow-to-cow, and even from day-to-day within any given cow. In short, the enzyme activity of rumen fluid from one cow can (and does) differ significantly from the enzyme activity of rumen fluid from another cow. Thus, while the test yields accurate aggregated results (because the differences in rumen fluid enzyme activity cancel each other out over a large number of samples), it is not sufficiently precise to determine the NDFD of any specific forage sample.
Neutral detergent fiber (NDF) is that portion of a forage that is insoluble in a neutral detergent solution. Neutral detergent fiber digestibility (NDFD) is conventionally defined as the digestibility of neutral detergent fiber as determined by the difference in NDF in a forage before and after in vivo or in vitro digestion as described in earlier. The NDF value reflects the total content of the cell walls of the forage. This is in contrast to acid detergent fiber (ADF), which reflects the cell wall portions of the forage that are made up of cellulose and lignin. NDF comprises the ADF fraction, plus hemicellulose (which is insoluble in neutral detergent solution, but soluble in acid detergent solution). Neutral detergent fiber values are important in ration formulation because they inversely reflect the amount of forage the animal can consume. As NDF percentage increases, dry matter intake generally decreases. In short, cows will eat more forage if the forage is low in NDF. NDF is the thus an accurate indicator of how much forage (on average) a herd will eat. For example, a high-producing dairy cow can eat about 1.1% of her body weight in NDF per day. If a grass forage has 50% NDF, a 1,300-pound cow will eat (on average) about 29 pounds of forage dry matter per day (1300×0.011/0.50=28.6). In contrast, that same 1,300-pound cow will eat (on average) about 36 pounds of forage dry matter per day of a forage containing only 40% NDF (1300×0.011/0.40=35.75). Because the cost of feed and the ultimate productivity of the herd are critical economic variables, the ability to measure NDFD accurately and precisely for any given type of forage is thus critical to maximizing profits from the on-going operations of dairy and meat herds. Likewise, the ability to measure NDFD is very important for commercial breeders of forage plants. Measuring NDFD of a forage or other biomass material is also important in the production of ethanol and other chemicals by enzymatic degradation and fermentation of biomass.
Norris et al. (1976) “Predicting forage quality by infrared reflectance spectroscopy,” J. Anim. Sci. 43:889-897 first recognized near-infrared reflectance spectroscopy (NIRS) was capable of predicting forage quality parameters, such as in vitro dry matter disappearance. Shenk et al. (1979) “Analysis of forages by infrared reflectance,” J. Dairy Sci. 62:807-812 acknowledged that NIRS had utility for commercial forage testing laboratories because NIRS instruments could offer rapid nutrient prediction. Shenk et al. (1979) also mentioned that two keys to success for NIRS prediction were: (1) the calibration samples must be representative of the population to be predicted; and (2) the reference laboratory data must be accurate. Abrams et al. (1987) “Determination of forage quality by near-infrared reflectance spectroscopy-efficacy of broad-based calibration equations,” J. Dairy Sci. 70:806-813 partially answered Shenk et al.'s (1979) first concern by determining a calibration set of at least 100 samples was necessary to approach the smaller error statistics of sample sets in excess of 400 forages.
The second key has been addressed by a number of studies, where acceptable reference technique accuracy and precision, as indicated by successful NIRS predictions, has been achieved for forage quality parameters such as NDF, in situ protein fractions, and in vitro dry matter disappearance. See Buxton and Mertens (1991) “Errors in forage-quality data predicted by near infrared reflectance spectroscopy,” Crop Sci. 31:212-218; Hoffman et al. (1999) “Prediction of laboratory and in situ protein fractions in legume and grass species using near-infrared reflectance spectroscopy,” J. Dairy Sci. 82:764-770; and Mentink et al. (2006) “Utility of near-infrared reflectance spectroscopy to predict nutrient composition and in vitro digestibility of total mixed rations,” J. Dairy Sci. 89:2320-2326. However, two attempts to calibrate NIRS to in vitro NDF digestibility data have failed: Andres et al. (2005) “Prediction of aspects of neutral detergent fiber digestion of forages by chemical composition and near-infrared reflectance spectroscopy,” Aus. J. Agric. Res. 56: 187-193; and Mentink et al., (2006) “Utility of near-infrared reflectance spectroscopy to predict nutrient composition and in vitro digestibility of total mixed rations,” J. Dairy Sci. 89:2320-2326. In their 2006 paper, Mentink et al. attributed the failure to a lack of precision with the in vitro ruminal digestion technique.
Thus, while the NDFD content of forage can be roughly predicted using near infrared spectroscopy (NIRS), there is a considerable loss of precision. See Combs (1998) “Using NIR to Evaluate Forage Quality,” Proc. of the 4-State Forage Feeding and Management Conference, University of Wisconsin-Extension, 129-135, Madison, Wis. Thus, current NIRS-based methods for predicting NDFD are less accurate and precise that the conventional in vivo method described by Goering & Van Soest, supra.
Neutral detergent fiber digestibility is an important parameter in modeling ruminant dietary digestion because the NDF fraction can be 30% or greater of dietary dry matter and digestibility can range from 30% to greater than 70% (Goeser and Combs, unpublished data). Thus, there remains a long-felt and unmet need for a method that determines fiber digestibility in general, and neutral detergent fiber digestibility in particular that is fast, accurate, and precise.